A carbon fiber bundle is a form gathering together a plurality single fibers consisting of carbon, and is used as a reinforcing material for thermoplastic resins or the like.
When used as a reinforcing material for thermoplastic resins, carbon fiber bundles are usually supplied being a form cut into lengths of 5 to 15 mm. During production of pellets obtained by kneading this carbon fiber bundles and a thermoplastic resin, it is necessary to supply the carbon fiber bundles in a fixed quantity into an extruder. In order to do this, the form stability of the carbon fiber bundles is important. A form that is not suitable can become a cause for discharge unevenness. Also, because a fixed extrusion rate cannot be attained, strand breakage occurs, whereby there is the risk of pellet productivity dramatically falling.
Materials known as long-fiber pellets are also attracting attention, in which carbon fiber bundles having a continuous fiber form are charged in the pellet production process. During this process, fluff or fly is easily formed on the carbon fiber bundle, or, the bundle is easily loosened, so that the handling is difficult.
Carbon fiber bundles can also be formed into a fabric and used as a sheet material impregnated with a thermoplastic resin, wherein the weaving quality of the carbon fiber bundles, the handleability of the cloth after being woven and the like are important properties.
For reasons as described above, to improve carbon fiber bundle handleability and the physical properties of a material into which carbon fiber bundles have been blended, it is conventional to use carbon fiber bundles which have been converged by a sizing treatment which deposits, for example, about 2 to 5 wt % of a sizing agent that is compatible with the matrix resin, such as an agent having an epoxy resin as a main component.
Examples of the thermoplastic resin used as the matrix resin typically include polycarbonate resin, nylon resin, polyester resin and the like. Recently, however, the number of cases of using a polyolefin-based resin has been increasing for reasons of recyclability and cost. Polypropylene resin in particular has been drawing attention in recent years.
Since polyolefin-based resins are basically nonpolar, their interfacial adhesion with the carbon fibers or glass fibers is extremely poor and the effects of improved mechanical properties as a reinforcing material cannot be adequately expressed in many cases. Countermeasures for this are known to include a method which improves adhesion by adding an acid modified polyolefin-based resin to the matrix resin, and a method which subjects the carbon fibers or glass fibers to a sizing treatment with a sizing agent constituted from a polyolefin-based resin and a silane coupling agent and the like. Another known method, as described in Japanese Patent Application Laid-Open No. 6-107442 (Patent Document 1), subjects the carbon fibers or glass fibers to a sizing treatment using a sizing agent having acid modified polypropylene as an essential component.
However, in the method which adds an acid modified polyolefin-based resin to the matrix resin, a large amount of acid modified polyolefin-based resin has to be added, whereby a product having excellent recyclability and cost effectiveness cannot be obtained. On the other hand, in the method sizing with a sizing agent containing a silane coupling agent, because carbon fibers do not have so many oxygen groups on the surface as compared with glass fibers, the effects for improving interfacial adhesion are rather minimal.
Further, although the method sizing with a sizing agent having acid modified polypropylene as an essential component described in Patent Document 1 achieves good interfacial adhesion as compared with a polyolefin-based resin, in the case of carbon fibers the effects were not sufficient.    Patent Document 1: Japanese Patent Application Laid-Open No. 6-107442